Nanoscience in Energy Sector

Transcription

1 Nanoscience in Energy Sector R. P. Deshpande B. Tech. Hon. (Elec), I.I.T. (Bom.); F.I.E Abstract: Nanotechnology is changing the energy generation and storage technologies in a significant way. Ultracapacitors are new generation capacitors with energy storage comparable to batteries, and very high power delivery. They seem to have potential even to replace batteries. UltraBatteries and newer generation batteries are storing larger amounts of energy with life longer than the equipment they serve. Nano based solar panels, solar paints, Dye Solar Cells and other developments are set to make solar power generation cheaper, and also are helping heat transfer for room heating etc. We may look forward to cleaner energy from nanotechnology, as also newer applications in electronic appliances like mobiles, cameras, PDA and a host of others. BIPV is gaining ground for better use of solar energy, with aesthetics. We will see nanotechnology making inroads in a big way in every energy sector. I. NANOTECHNOLOGY AND ENERGY Nanotechnology is science at the smallest scale, and allows manipulation of the fundamental building blocks of all matter atoms. By designing materials at nano level, scientists can create super materials with incredible new chemical, electrical and physical properties never before thought possible. Real potential of nanotechnology lies in the field of energy. By generation, storage, transmission and use of energy at the nanoscale, we will get energy far cleaner, more efficient and therefore cheaper, than any of the fossil fuel or alternative energy technologies of today [1]. II. MATERIALS USED FOR ENERGY APPLICATIONS The base materials commonly used are activated carbon, a tiny nanotube structure or aerogels to form a highly porous solid. Fig. 1 shows some representative materials from each class. Activated carbon is highly porous amorphous particulate matter. Nanotubes are typically of diameters the order of 5 nm, or 1/10,000 times the diameter of a human hair. These are spaced about 5 nm apart, and may be about 100 µm long. Graphene are one-atom type bound structures with conducting particles interspersed. Aerogels are feather-light, transparent high surface area solids. In addition, several other nanomaterials are in use and more are under development for varied applications. Activated carbon Carbon nanotubes Figure 1: Some materials used in batteries and Ultracapacitors [11] Graphene Aerogel A. ACTIVATED CARBON Activated charcoal, produced from coconut shells, palm seeds, wood and some other materials is an extremely porous, "spongy" form of carbon with an extraordinarily high specific 2 surface, area 1 gram has a surface area of roughly 250 m about the size of a tennis court [3]. It is typically a powder made up of extremely fine but very "rough" particles, which, in bulk, form a low-density heap with many holes. As the surface area of even a thin layer of such a material is many times greater than a traditional material like aluminum, many more charge carriers (ions or radicals from the electrolyte) can be stored in a given volume. Microfine Carbon powder consists of three types of particle sizes: Microfine Carbon powder consists of three types of particle sizes: Micropores - < 20 Å wide, Mesopores Å wide and Macropores - >500 Å wide. Pore size is important for electrolyte contact, and a mixture of the three sizes are used for best results. B. CARBON NANOTUBES CNT are increasingly used in ultracapacitors due to their high chemical stability and high conductivity [3]. They have light mass and large surface area, and a typical Nanotube Structure 2 has a High tube density of the order of CNT/cm, with tube length of μm. They can store energy density higher than 60 Wh/kg, (comparable to NiMH batteries.), a power density greater than 100 kw/kg (3 times higher than batteries), and capacitors made from these have a lifetime of longer than 300,000 charge discharge cycles. 53

2 C. GRAPHENE These are single atom thickness sheet lattice with binder free active layers, giving more active area in the range of sq. m. /g. [3]. They pack a power density of 20W/cc, with a specific capacitance of about 380 F/g. D. AEROGEL - THE NEWEST MATERIALS Carbon aerogel [3] are so porous and lightweight that they are up to 99.8% air, with densities ranging as low as 1.9 mg/ cc, are feather light, but can support up to 8000 times its weight. They can have surface areas up to 3000 sq. m. or more per gm. These can be produced as thin films. The energy produced in carbon aerogel capacitor is 325 KJ/kg, about 70 per cent of what is provided by Lithium polymer battery. Aerogel can also be made using materials other than carbon, and trials are in progress on such materials. We will now proceed to review the use of nanotechnology in energy field. III. ELECTROCHEMICAL CAPACITORS (EC): Till now, capacitors were known to be either electrostatic or electrolytic. A new class has appeared on the scene- Electrochemical Capacitors, whose basic function would be storage and supply of electrical energy. It is variously known as: electrochemical (EC) capacitor, electrochemical double layer capacitor (EDLC), double layer capacitor (DLC), ultracapacitor, supercapacitor, digitized energy storage device (DESD), Gold capacitor, etc. These widely used but little known capacitors hold out good promise as energy storage devices of future [2]. Nanotechnology has made possible materials with extremely high porosity, opening huge potential in the development of these capacitors. Farad is no longer too large a unit, and values of EC capacitors are generally in F or KF. The EDLC (or Ultracapacitor) principle is shown in the figure below. Two active porous carbon electrodes may be used, with electrolyte filling the remaining space, and a separator used between these two sections. Two layers of dielectric are thus formed, and hence the name Double Layer Capacitor or EDLC. The carbon electrode is made from activated carbon particles painted or rolled on a metal foil collector. Figure 2: Electrochemical double layer capacitor [2] The high energy stored in ultracapacitors is their biggest asset. Conventional capacitor has energy density of 0.15 Watt-hour (Wh) per kg, whereas EDLCs may store up to 15 Wh. The stored energy levels are comparable to that of a battery. Efforts are on to increase the density, when it will store nearly half the energy compared to a similarly sized Li-ion rechargeable battery. Li ion capacitors already store up to 21 to 25Wh/Kg. Targets are pointing to the possibilities of approaching 40 Wh/Kg or more in near future. In fact, one manufacturer in Japan is hopeful of increasing this level up to a maximum of 75 Wh/kg. By comparison, Ni-MH battery packs around 45 Wh/kg. While present energy densities are much below that of batteries, their power delivering capabilities of up to 2000 W/kg are ten times that of batteries[2] Theses capacitors offer an eco-friendly alternative for energy storage, and can work with batteries for backup power peaks, or even work independently as standalone storage devices. They are being used in millions around the world already, and will become as indispensable as batteries in near future. The well-known formula for capacitance is C = Є0 K A / d Where K= dielectric constant of separator medium between electrodes. Nanotechnology has enabled d, the dielectric thickness, to reach nanometer scale, and areas of sq. meters / gm. The ratio A / d has reached a level of Dielectric in Electrochemical capacitors is formed as an extremely thin layer at the contact surface of two different electrode materials. One of the electrodes is highly porous solid (usually activated carbon), and a liquid electrolyte filling it through increases the effective contact area tremendously. An extremely thin oxide layer formed at the interface acts as the dielectric. This allows very high capacitance values, even a couple of thousand Farads, to be accommodated in capacitors. Capacitors up to 100,000 Farads have been made in a single unit. Energy stored in capacitors is expressed as ½ CV2, and one may calculate the tremendous amount of energy that can be stored in ultracapacitors. They combine high energy potential of batteries with high energy transfer rate and fast recharging capabilities of capacitors [2]. Voltage rating of EC capacitors is solely limited by the decomposition potential of its electrolyte. Voltage ratings of EC capacitors are typically between 1.3 to 2.8V, while 2.3V to 2.7V are more common. These basic cells of EDLC are connected is series parallel combination to increase the voltage ratings for different applications. EDLC (Supercapacitors) have power densities up to hundred times that of batteries (Fig. 3), although total stored energy is far less. They can give large bursts of power for short duration, required in several applications like vehicle starting, saving the battery from deep discharging, thereby increasing the battery life 4-5 folds. They can do this since no electrochemical process is involved, the stored energy being in electrical form in a 54

3 Figure 5: Ultracap for bus- 28F /450V / 910 WH [11] Figure 3: Energy and power density comparison of energy sources capacitor, available for immediate use. New materials for anodes and experiments with various electrolytes, including those similar to ones used in Li-ion batteries are helping improve energy densities beyond what has been possible till now. In fact researchers are even hopeful of replacing batteries with EDLCs for most applications, and energy densities higher than batteries seem as a possibility. Activated carbon electrodes of EDLCs are made of high porosity carbon having active area of the order of 1000 to 2000 square meters per gm. in particulate, plastic bonded or film form, in contact with a current collector. Carbon may be pasted on aluminium foil and wound on high speed machines. While winding pressure is enough for cylindrical capacitors, high pressures are applied to cells for keeping continuous contact of electrodes with current collectors. A. ULTRACAPACITOR APPLICATIONS Automotive & traction- In vehicles, ECs find use in Regenerative braking, Electric drive, Hybrid vehicles, Battery backup, cold weather starting, Start-stop systems (at signals), Power steering (needs ~ 2KW power), Air conditioning, Accessories radio, horn, etc., EC based subsystems improve fuel efficiency & reliability. With million passenger vehicles p.a. produced worldwide, prospects for ultracapacitor have tremendous potential. Further, ECs are being seen as a replacement of batteries as a main power source in many vehicles, apart from more common use in electric vehicles as backup power with batteries or fuel cells. Already buses running in Shanghai on ultracapacitors as the lone power supply (recharged every third stop in 1-2 min.) are a success. Their use in cars along with batteries helps reduce battery size and is economical in the long run. Table 1 shows a comparative advantage of EDLC plus battery combination. Figure 4: Ultracapacitor shapes & sizes[11] Table1: Battery + capacitor in vehicles [11] Electronic and low power applications -Ultracapacitors up to 4 Farads V, in coin (button) type / cylindrical shape are used in small high freq. devices to reduce battery size. They serve in memory 55

4 function in laptops, mobile phones, radio tuners. Electronic diaries, PDA, Pagers, data loggers, vending machines use them as battery backup and for peak power assistance in digital cameras. ECs further serve as energy storage in toy motors, tools like portable hand drills and screw drivers, where they can be quickly charged whenever required. Grid energy storage - Ultracapacitors serve as grid energy storage, where they serve better than batteries for peak shaving, frequency and voltage regulation, system stabilization through short time surge protection. They also serve in UPS systems to take care of load for short time till alternative supply takes over, or a generator starts. Further, they find use in renewable energy storage systems. On account of their reliability under all weather conditions and wide temperature range, they find extensive use in military applications as backup power for electronics in military vehicles, fire control systems in tanks and armored vehicles, airbag deployment, power/memory hold-up emergency handheld radio, GPS guided missiles and projectiles, high power discharge for naval warfare, cold engine start, peak power: communication transmission, UAV (Unmanned Aerial Vehicles), Radar and many other places. Figure 7: (a) FURUKAWA UltraBattery Figure 7: (b) MITSUBHISHI Li-ion UltraBattery Energy Density: WH/Kg Figure 6: Peak power application model with ultracapacitor energy storage [11] B. HIGH CAPACITY BATTERIES Batteries are needed to store electrical energy. Their applications include mobile phones, walkmans, as also home or village power supply in remote areas and in backup systems in case the grid fails. Trend is towards rechargeable batteries, both dry and wet types. Dry batteries are mostly Lithium based, (e.g. Li-ion batteries). Wet batteries are based on metal hydrides (where hydrogen is the chemical energy carrier), or carbon nanotubes. Lead Acid UltraBattery [12], is a hybrid between lead-acid battery and ultracapacitor, electrodes of which are integrated into a single device with two terminals. UltraBattery technology is being mainly developed for two major applications: low emission transport (specifically hybrid electric vehicles), and renewable energy storage from wind and solar. Applications in electronic, computer and mobile have also seen advent of UltraBattery via Lithium technology. already started to impact storage technology. It might someday allow far more powerful, more efficient and less expensive solar and battery storage technology. Battery companies have indicated that by using nanotechnology to design new anodes and cathode materials, they are able to greatly increase the amount and rate of energy that can be transferred to a battery, and reduce the recharge times significantly as a result. C. SOLAR ENERGY Nanotechnology is set to revolutionize the solar energy sector, with promise of getting cheap and clean energy. Solar energy can be produced by every household on rooftop or farm. If we add the energy generated by Sun's invisible rays (IR), the potential is immense. In today's times, clean energy is becoming a huge business in itself. Many developments in clean energy revolve around strategic applications of nanotechnology. Use of Nanoparticles in the manufacture of solar cells reduces manufacturing costs as a result of low temperature process similar to printing instead of the high temp. Vacuum deposition process typically used to produce conventional cells made with crystalline semiconductor material. Reduced installation costs are achieved by producing flexible rolls instead of rigid crystalline panels. Cells made from semiconductor thin films will also have this characteristic. In the long term nanotechnology versions should lower the cost and, 56

5 using quantum dots, should be able to reach higher efficiency levels than conventional ones. D. SOLAR CELLS The most efficient solar cells are made up of layers of expensive crystalline silicon. These have chemicals added to encourage photons to liberate electrons, which pass between layers to create a current. Nanotechnology can help by increasing the size of interfaces by creating incredibly bumpy surfaces. This allows more electrons to pass, increasing the amount of electricity produced, and reducing the energy cost. One kind of new solar cell that does precisely this is the dyesensitized solar cell, or Graetzel cell [7]. These solar cells consist of a highly porous layer of TiO2 nanoparticles, coated in a molecular dye (these use sunlight to mimic chlorophyll and create energy). These are more flexible, can work in higher temperatures and operate even in low-lighting conditions; but the key benefit is the size of contact surface and the potential to create energy. Its porous nature allows much larger surface area (over a thousand times) covered by the dye than the apparent area of the cell, as per co-inventor Michael Graetzel at the Swiss Federal Institutes of Technology, in Lausanne. Organic Grätzel solar cell consists of a 10μm thin layer of TiO2 particles, 20 nm in diameter. Organic dye molecules are adsorbed in the pores between the TiO2 particles, surrounded by an electrolyte fluid. The cell is completed by two transparent conducting electrodes, and a catalyst. The cells are not yet commercialized as they need further improvement in energy density. The latest victory in energy efficiency comes from scientists at the University of New South Wales in Australia [14], who have produced a multi-cell combination that converts 43% of sunlight into electricity, bettering the previous record of 42.7%. E. HEATING PANELS: Solar heating panels available in the market today are usually silicone panels, which capture 67.4% incident light. With nanotechnology, industrial and electrical manufacturers can now produce a coating for solar panels using nanorods that can capture 96.7 % light. New process developed at California Institute of Technology may use only 1% of material used in conventional cells [13]. Installing a nanorod on solar panel can give a lot of benefits like lower costs, and they accept solar rays from any angle. F. SOLAR PAINTS Generating electricity by the paint on your house/ office is going to be a reality soon. Nano-solar paint in development works like a solar panel [8], at a much lower cost. It is a sunlight absorbing paint coated onto the surface of aluminized mylar. This conducts electricity. It is coated with a clear layer of transparent Indium tin oxide that covers the paint and also conducts electricity when exposed to sunlight. This means lower costs, higher efficiency, and fewer panels needed to do the job. As technology improves, oil energy dependence can be reduced. BIPV may use minuscule size of the solar cells. They can be can be mixed in liquids and sprayed onto surfaces. Nanosolar is using such a solution ( ink ) to create rolls of bendable solar cells. Someday, spray-on cells could be incorporated into paint and product coatings. In other words, your new exterior siding, sheet metal roofing or windows could be made with electricityproducing solar cells. Conventional, aluminum-framed solar panels are usually an addition to the average home. Future solar components, instead of being on the roof, will be roof itself [8]. Already, solar shingles (mostly solar roof tiles) are lining many rooftops, keeping the rain out while taking the sunshine in. Companies like Nanosolar are printing solar cells [6] onto thin rolls of foil. Researchers are working to commercialize spray-on solar cells that could become the most energetic paint. It will be faster, use less material and at a lower cost. G. DYE SOLAR CELL (DSC) Justin-Hall Tripping of Nanotech technologies, detailed an exciting and practical new technology- Transparent Solar Panel Windows. Covered with a clear film made up of engineered electrons, these windows would harness solar energies traditionally lost to thermal processes. This would supply energy to run household utilities, and also regulate temperature, storing excess heat in hot times and distributing it during cold temperatures. This DSC window installation is the first in South Korea and is a beautiful showcase building for DSC technology. If successful, Seoul City will expand this initiative through the application of DSC BIPV into other buildings Figure 8: DSC Windows 57

6 The (Dye Solar Cell) DSC window installation (Fig. 8) is the first in South Korea and is a beautiful showcase building for DSC technology. If successful, Seoul City will expand this initiative through the application of DSC BIPV into other buildings. DSC technology emulates the natural process of photosynthesis. Using a layer of Ti, electrolyte and ruthenium dye sandwiched in glass, DSC technology generates electricity when light striking the dye activates electrons absorbed by the titania to become an electric current. The technology produces electricity more efficiently at lower cost even in low light conditions [4], and can be directly incorporated into buildings by replacing conventional glass panels or metal sheets rather than taking up roof or extra land area. IV. CONCLUSION Nanotechnology has contributed to major advances in energy storage systems, including batteries and ultracapacitors. Renewable energy plants will become common once large energy storage become practical. Solar energy collector systems have become more efficient. Solar panels and collector systems can now be sprayed like paint. Solar panel efficiency is improving with material development to convert IR radiations. BIPV is gaining ground for better use of solar energy, with aesthetics. In short, we will see nanotechnology making inroads in a big way in every energy sector. REFERENCES [1] Nanotechnology helps solve the world's energy problems: Ineke Malsch, Nanotechnology Now. [2] Ch. 13, Ultracapacitors: Capacitors-Technology and Trends, R. P. Deshpande [3] Nanostructured Carbons: Double-Layer Capacitance and More: Patrice Simon and Andrew Burke, The Electrochemical Society Interface- Spring [4] More Efficient Dyed Cells Offer Hope for Cheap Solar Windows: David Biello, Scientific American [5] DSC: A beautiful future emerges in Korea: DYESOL, Australia, Newsletter,2012. [6] Nanosolar flexible solar cells printed on aluminum foil: Designboom, [7] Grätzel Cells, Regenerative Photoelectrochemical Cells: Electropaedia [8] Solar Paint Converts Light to Electricity: Scientific American, [9] Silicon Nanorods Could Increase Solar Cell Efficiency, Cut Cost: Sandra Henderson, Solar Novus Today, 2012, [10] Development in Supercapacitor: Marin S. Halper, James C. Ellenbogen [11] Ultracapacitors- The Energy Storage Device, Presentation by R. P. Deshpande at CAPACIT,2010. [12] UltraBattery: CSIRO, Australia Newsletter [13] New Silicon Nanorod Solar Cells Use 99% Less Material: Ariel Schwartz. [14] Australian Scientists Develop World's Most Efficient Solar Cell: Ariel Schwartz. 58